This invention is directed to an electronic wristwatch having a sound-emitting device or alarm therein, and, in particular, to an electronic wristwatch including a piezoelectric element coupled to the case back cover of the wristwatch to form a piezoelectric buzzer.
In conventional wristwatches, an alarm or other sound-emitting device can be incorporated in the watch casing which is selectively driven by a driver in the watch circuit. One particular type of alarm utilized in wristwatches is the piezoelectric buzzer. The piezoelectric buzzer generally has a bi-morph construction and has the structure where the vibration plate, such a metal plate or the like, and the piezoelectric element are attached to each other with a binding agent or the like. The piezoelectric element can be attached to the cover glass of the watch so that the cover glass acts as the vibration plate of the buzzer. Alternatively, the piezoelectric element may be mounted in a centralized portion of the watch case wherein a separate vibration plate is provided therefor as in U.S. Pat. Nos. 3,879,931 and 4,180,970. In another embodiment, the piezoelectric element can be attached to the back cover or case back which acts as the vibration plate, as shown in the Official Japanese Gazette No. 33977-77.
The piezoelectric buzzer is simply constructed and can produce a loud sound with comparative low power consumption. Accordingly, in recent years, various products such as timekeeping devices, incorporating piezoelectric alarm constructions are on the market. However, in wristwatches, where the piezoelectric buzzer is attached to the case back of the wristwatch, the case back acting as the vibration plate of the piezoelectric element, it is found that the sound pressure level or volume of the buzzer is substantially decreased when the wristwatch is worn as compared to when it is not worn since the wrist of a wearer presses against the case back.
The shape and construction of the case back of a wristwatch where the piezoelectric element is attached thereto was only designed with considerations of strength and design in mind. Thus, the natural frequency of the case back was not considered.
Accordingly, a very thick case back was used, for example in a water-resistant watch requires high pressure resistance. Alternatively, a very thin case back was used in a thin type watch with only design considerations in mind. Thus, consideration was not given to constructing the back cover in such a way that the natural characteristics thereof could be utilized in connection with a piezoelectric element attached thereto so that the volume of the buzzer does not substantially vary when the watch is worn from when it is not worn.
Accordingly, an electronic wristwatch including a piezoelectric element attached to the case back which acts as the vibration plate of the buzzer wherein the buzzer volume does not substantially vary when the watch is either worn or not worn, is desired.